(not in any order of preference)
1. Ask them nicely to let you out, which unfortunately means waking them.
2. Not recommended, it is unhealthy.
3. Not recommended, it is unhealthy.
4. Not recommended, it is unhealthy.
5. Climb over them
6. Summon the F/A using the call button for help.
7. if you are a guy, go into the air sickness bag if you can.
If you wait too long you may run into the 30 minute sitdown before landing and be unable to go until you get into the airport.
This problem will be exacerbated for aisle seat occupants if airlines don't want people congregating near the lav's.
Offer to let them slide over and you take the aisle seat when you get back.
Travel tips:
http://members.aol.com/ajaynejr/travel.htm